1. Field of the Invention
The present invention relates to a process for the preparation of methyl fatty acid esters starting from natural oil or fat by transesterification with methanol in the presence of a catalyst.
2. Description of the Prior Art
The methyl esters derived from natural oil or fat have become important in the oleochemic industry as fuel or as raw materials for the preparation of several derivatives, such as fatty alcohols or fatty alkanolamides. These methyl esters can be prepared by esterification of fatty acids but i the conventional process used for the preparation of these methyl esters is the catalytic transesterification or methanolysis of triglycerides, i.e. an ester substitution, with an excess of methanol. During the transesterification, which is effected according to the following reaction scheme: ##STR1## the glycerol molecule in the triglyceride is replaced by three methanol molecules. The preparation of methyl esters by transesterification is described in "Manufacture of Fatty Alcohols Based on Natural Fats and Oils" from U. R. Kreutze, JAOCS, vol. 61, No. 2 (February 1984), pages 343-345 and in "CEH Marketing Research Report--Detergent Alcohols" from R. F. Modler et al., 1990, Chemical Economics Handbook, SBI International.
In these known processes, a good yield of fatty acid esters obtained by the use of an alkaline catalyst, such as sodium or potassium hydroxide in a solution in methyl alcohol or sodium methylate.
The use of an alkaline catalyst involves however several drawbacks:
the alkaline catalyst is very corrosive and in view of its high pH (in the range of 8 to 10), the reaction medium gives also very corrosive by-products; consequently, the equipment used for the process has to be in stainless steel; PA1 an additional neutralisation step of the free fatty acids contained in the oil or in the fat is necessary together with a separation of the so-formed soap; PA1 an additional step for refining the glycerol containing waters and for separating the salt is also necessary; PA1 the recuperation efficiencies of the methanol excess and of the glycerol are low.